A jib furling system on a sailboat is a mechanism to roll up, or “furl” a jib either to reduce sail area or to store the jib. Typically, a furler system consist of a bottom drum, an aluminum or plastic extrusion and a top fitting. The extrusion has a variety of slots vertically that the lull of the jib slides up to secure. The line that hoists the jib is referred to as the jib halyard. The head stay or forestay, usually a wire rope, that secures the mast is located inside of the extrusion. There are, in general, two types of halyards internal to the extrusion and external.
An external halyard commonly uses the sailboat's jib halyard, from the mast, to hoist a shuttle that slides up the extrusion. The head of the jib is secured to the lower part of the shuttle. With the lower part of the jib (tack) secured to the lower drum, this tensions the sail. The problem with this design is that when the jib is furled, the shuttle has to rotate which causes the upper portion of the halyard to wrap around the top of the extrusion. This is referred to as halyard wrap. To compensate for this, many manufacturers incorporate a swivel in the top fitting.
On systems with internal halyards, the issue of halyard wrap is not a problem. The halyard rotates completely as the sail is furled. In example of FIGS. 1A and 1B, the halyard is attached to the head of the sail, goes up to and around a sheave, if present, then down through the front extrusion groove/slot. This would work fine except for the fact that the headstay is in the middle of the fitting. As such, the halyard, and sheave if present, cannot be correctly aligned with the front slot and the rear slot that locates the jib. While it may not seem significant, this can offset the top sheave 40 to 50 degrees. This due to the geometry and very little space. This offset or rotation of the sheave has a number of issues including: (i) Significant halyard and sheave wear, (ii) Increased friction, and (iii) Hoisting of the sail from an angle so as to increase the sail/extrusion friction and wear.